Hybrid junction mixers are known to be extensively used in various microwave and millimeter wave balanced-circuit configurations, including balanced mixers single-sideband modulators, phase and amplitude monopulse comparators, frequency multiplexers, and constant impedance filters.
Several useful forms of hybrid junction mixers are well known, including the hybrid ring or "rat race," and each form is known to be able to be implemented in waveguide, coaxial line, or stripline media.
The basic hybrid ring comprises a closed loop transmission line essentially six quarter wavelengths in length. Four ports on the ring are spaced 60.degree. apart geometrically, and one quarter wavelength apart electrically. For an ideal hybrid ring, a signal input to port 1 divides equally between ports 2 and 4 with a 180.degree. phase difference, whereas a signal input to port 3 divides equally between the same ports 2 and 4 with a zero phase difference.
The prior art is replete with variations of the basic hybrid ring. One such prior art disclosure describing a hybrid ring as a mixer is contained in U.S. Pat. No. 4,031,472, the construction of which is said to avoid certain defects of the earlier devices.
The present invention differs from the prior art in several ways, one of which is that it relates to a millimeter wave suspended substrate mixer with a 180 degree hybrid, with a highly advantageous arrangement for extracting the intermediate frequency (IF.).
Others in endeavoring to deal with suspended stripline or other planar transmission lines have found it necessary to use a quarter wave stub for providing radio frequency and local oscillator ground, and this had the unfortunate characteristic of limiting bandwidth, and in addition requiring the use of jumper wires for extracting the I.F. signal.
For example, in a certain quartz suspended substrate circuit for radar applications, phase detectors and phase lock loop mixers were often found to occur in awkward and tight spaces. The I.F. had previously been extracted with a network of capacitors, resistors, and lines.
It was in an effort to overcome the disadvantages of these and other prior art techniques that the present invention was evolved.